Power has now been restored after another blackout hit northern parts of Sydney's central business district this morning.

Kylie Yates, a spokeswoman for Energy Australia, says the outage affected buildings between Kent and Hunter Streets from Market Street up to the Rocks, and power was restored by just after 11:00am (AEST).

"Power was interrupted to the northern part of the CBD for about 40 minutes this morning," she said.

"We believe it was a fault on a cable that supplies that part of the city, which triggered our protection equipment and shut down power to that part of the city."

She says the cables run from Lane Cove to a substation in the city, and that a fault in one of the cables triggered protection equipment that shut down all the power.

"The reason that there have been three interruptions to this part of the city is due to a larger fault on our protection equipment," she said.

"This equipment is designed to shut down a part of the electricity network when a fault occurs, and this equipment is shutting down a wider area than we would expect. So that's something we're investigating, and have been investigating as a priority."

Police say that they attended at least two cases of people trapped in lifts, and ABC Sydney local radio had reports of people being evacuated from numerous office buildings in the area.

The Australian Fashion Week show was also thrown into darkness, and the RTA says 27 sets of traffic lights were blacked out during the outage.

Opposition blames neglect

The state Opposition's energy spokesman, Duncan Gay, says the blackout shows just how fragile Sydney's electricity grid is at the moment.

"The real reason that we are having this number of blackouts is 14 years of neglect," he said.

"The Government is trying to squeeze extra money out of the people that are paying the bills. The problem is that the Government has failed to maintain the infrastructure to a decent standard."

However, Energy Australia says it has invested a lot of money upgrading the city's power supply, and will be replacing the faulty protection equipment.

"We have been working on replacing that equipment, we have been planning a replacement of that protection equipment so that will give even greater security to the part of the CBD that's affected," Ms Yates said.

However, she could not rule out further power outages in the meantime.